1. Field of the Disclosure
The present invention generally relates to a method for manufacturing a photomask and, more particularly, to a method for manufacturing a phase shift mask using electron beam lithography.
2. Brief Description of Related Technology
As semiconductor devices have become more highly integrated, the size of patterns formed on semiconductor substrates has been gradually reduced. Further, as semiconductor devices have a tendency toward high integration and high density, various methods have been developed to form fine patterns. From among the methods, a pattern-forming method using a patterned phase shift mask (including a phase shifter) has been used to increase resolution.
A phase shift mask exposes a pattern having a predetermined size by using interference or partial interference of light to increase resolution or a depth of focus. That is, a difference in the phase of the light occurs, according to the existence of a shifter, when light passes through a mask substrate. The light having passed through the shifter is out of phase with the light having passed through a non-shift area of the substrate. Because the light having passed through only a transmitting part is out of phase with that of the light having passed through the shifter, the shifter is located at the edge of the mask pattern, so that the intensity of the light is offset at the boundary of the patterns and, thus, the resolution is improved.
For example, such a phase shift mask includes an alternating phase shift mask. The alternating phase shift mask has a phase shift pattern, which is formed by etching a transparent substrate by a predetermined depth, to shift the phase of the light transmitted through the transparent substrate. To manufacture the alternating phase shift mask, a quartz substrate is coated with an opaque layer, which layer functions as a mask and resist. The coated substrate is subjected to exposure and development processes using an electron beam to form a resist pattern. Thereafter, the opaque layer is etched by using the resist pattern as an etching mask, and the substrate is etched to a predetermined depth, using the etched opaque layer as an etching mask, to form a phase shift pattern.
However, according to the conventional method for manufacturing the phase shift mask as described above, the manufacturing process is complicated and lengthy. In particular, the substrate must be etched to a predetermined depth to form the phase shift pattern. However, it is difficult to precisely etch the substrate to a desired depth and width.